FIELD OF THE INVENTION
The present invention relates to an on-line sheeter of a printing system and to a method of changing a length of cut and, more particularly, to an on-line sheeter of a printing system which is designed so that it can cut running printed sheets discharged from a web-fed rotary gravure press or a web-fed rotary forms press, at a predetermined length while on-line with the press, and method of changing the predetermined length of the cut. The length of the cut is length of the sheet between two cuts.
Up to the present time, when exchanging plate cylinders and changing the length of cut in the operation of a web-fed rotary press for gravure or forms, the printing systems have been arranged in such a way the desired section of sheet could be obtained by providing a plurality of folding machines corresponding to the length of cut for folding various running sheets. The folding machines comprise a plurality of folding cylinders positioned on the running line of the sheets. Each of the folding cylinders includes a cutting blade for cutting sheets, a needle for supplying sections of sheet being cut or a striking blade, and a gripping apparatus introducing the section to a stacker. Alternatively, variable folding machines sometimes prepare sections of the sheet.
However, in such conventional systems, printed sheets sometimes cannot be fed to a folding machine. For example, in a case of label printing where the sections of the sheet are not fixed, the printed sheets cannot be discharged as a section. Such printed sheets are temporarily rewound on a rewinder, then each of desired lengths of cut is set in an off-line sheeter, and the sheets are cut one by one to be stacked. Such a sheeter introduces printed sheets supplied from a roll of rewound sheets to a flying knife through a guide roller, a compensate roller, and a drawing roller, and then cuts the sheets at predetermined sizes and introduces them to a stack section to pile. In this case, an adjustment of the variation in the length of cut is carried out by modifying a press length to a draw roller through adjustment of a compensate roller position and by controlling a rotating speed of the draw roller. Controlling the rotation of the draw roller is control of the circumferential speed, which speed is manually or automatically controlled.
The above-mentioned conventional sheeter, however, is arranged off-line and independent of a printing line. It performs a cutting operation after loading a rewound roll of the printed sheet, therefore, its working effectiveness is very low. Moreover, the sheeter itself needs to be changed to a model corresponding to its length of cut when changing the length of cut with an exchange of a printing plate cylinder (e.g., changing size A to B). Because of the above-mentioned disadvantages, there has been a problem that the conventional printing system is not only unable to cut running printed sheets discharged from a web-fed rotary gravure press using an on-line sheeter. Further, the system needs to be equipped with a sheeter of a model corresponding to the new length of cut when the length of cut is changed. To do otherwise, the system has to employ a sheeter which can vary a length of the cut while on-line. To do so a draw roller is controlled, using a separate driving power source, to inhibit the variation of a length of the cut. However, the printing speed of the press varies. Thus, controlling the rotation of the draw roller to adapt to the variation is very difficult, and the conventional printing system has a disadvantage that it cannot control the variation of the cut as a result.